Switching mode power amplifier with load isolation

ABSTRACT

A power amplifier device includes first and second pairs of semiconductor switches, transformers, and a zero-crossing detection circuit for detecting a zero voltage crossing of an analog input signal. The switches of the first pair receive a respective positive and negative component of the input signal. The transformers store energy from the positive and negative components, respectively. Each transformer releases accumulated energy when the respective switch of the first pair turns off The switches of the second pair have opposite switching states and are connected between a respective transformer and a load, e.g., a transducer, speak, or motor. Each switch receives released energy from the respective transformer. A switching state of each switch of the second pair changes in response to a detected zero voltage crossing of the input signal to transfer the released energy to the load. A system includes the device and the load.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 62/201,347 filed on Aug. 5, 2015, which is herebyincorporated by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates to a switching mode power amplifierhaving a switching function that is isolated from a load.

BACKGROUND

Power amplifiers are electronic devices operable for amplifying an inputto a level that is suitable for driving a load, such as an audiospeaker, a transducer, or an electric motor. For alternating current(AC) input signals, the impedance of the load is determined by thefrequency of the input signal as well as the resistance, capacitance,and inductance of the load. Power amplifiers typically include a powersupply, an input stage, and an output stage. The power supply may be ofthe linear type or the switching mode type, with the latter typeproviding higher relative energy efficiencies. There is a particulartype of power amplifier known as a switchmode power amplifier. This typeof power amplifier may also contain a power supply of the linear type orof the switching mode type.

A switch mode power supply may also be used to modulate a signal in aparticular type of switchmode power amplifier. The output stage,controlled by the input stage, applies precisely timed pulses to a loadto amplify the relatively weak input signal and thereby generate anoutput signal having a power level sufficient for driving the load. Suchpower amplification may be provided via a transformer. Conventionally,the power supply remains connected to the load, with the impedance ofthe load tending to lower the overall efficiency of the amplifier.

Of the power amplifier types noted above, switching mode-type poweramplifiers in particular operate by applying electrical power to theload. For instance, fixed or variable width pulses representing adesired signal may be provided by a fixed-amplitude power supply atprecise intervals. Alternatively, fixed or variable width pulses may beprovided from a variable power supply. The frequency of the switchingpulses is significantly higher than that of the desired output signal sothat energy from the switching pulses can be integrated over time toreproduce the desired signal. A high switching frequency is alsodesirable in order to simplify the task of filtering out undesiredenergy produced at the switching frequency. Depending on the impedanceof the load, higher power required at the load may require a highervoltage. Thus, the power supply used for high-impedance loads mustproduce higher voltage levels relative to voltage levels used with lowimpedance loads.

The switching speed of a typical solid-state semiconductor switchoperating at higher voltage levels is relatively slow compared to thespeed of a switch used in lower voltage devices. The parasitic seriesloss of a high-voltage semiconductor switch is also higher than a lowervoltage device. As a result, it may be difficult to precisely time thedelivery of signal pulses to a given load. Precisely timed delivery ofsignal pulses is important to maintaining high signal fidelity. If theload is connected during the switching interval, the finite switchingtime and parasitic switching loss of the semiconductor switch willresult in increased distortion observable at the load.

A switching mode amplifier may employ a boost transformer. If the boosttransformer also carries the desired signal in addition to the switchingsignal, the boost transformer design is limited by the relationship tothe frequency of the carrier signal, i.e., the carrier frequency. Priorart designs without load isolation require a close relationship betweenthe signal frequency and the magnetic design. For instance, if thedemodulated signal frequency falls within an example frequency rangeused for driving audio applications, the magnetizing inductance must behigh with respect to the demodulated frequency involved and thereforethe transformer must use a magnetic core having a high magneticpermeability in order to be sufficiently compact for practical use.Furthermore, the saturation profile of any magnetic material used in theconstruction of the transformer is directly related to the demodulatedfrequency or carrier frequency. This relationship to the demodulated orcarrier signal significantly limits the types of magnetic material thatcan be used, and also limits the choice of upper switching frequency.This limit on upper switching frequency is due to accumulation of eddycurrent losses and other factors.

Modulation techniques such as pulse width modulation (PWM) ordelta-sigma modulation (DSM) may be used for the carrier function in anamplifier. Delta-sigma modulators, which convert a high-resolution inputsignal into a high-frequency signal having a relatively low resolution,e.g., a 1-bit pulse train, are particularly useful when the ratio ofmodulation to the carrier signal is relatively low, for instance a ratioof less than 10. DSM can be used to shape quantization noise and therebyreduce noise within the frequency range of the input signal. PWM can beused when the ratio of modulation to the carrier is relatively large,e.g., greater than 10. PWM may be easier to implement for the case ofhigher ratio of modulation-to-signal, but requires a faster switchingspeed than DSM. The present state of the art attempts to minimize theeffects of finite transition time of the transistors used in the designof power amplifiers of the types using high-speed switching.

SUMMARY

An improved switching mode power amplifier device is disclosed hereinthat provides load isolation. In the disclosed configurations theimpedance of the load does not lower the efficiency or accuracy of theamplifier device in the conventional manner, i.e., since the load isisolated during the switching interval. Additionally, the amplifierdevice is scalable to different loads over a very wide carrier frequencyrange in different applications, limited only by switching speeds andparasitic terms of semiconductor devices and magnetic devices used inthe construction of the amplifier device. That is, a power supplyconventionally remains connected to the load, with the impedance of theload tending to lower the overall efficiency or accuracy of theswitching power amplifier device. Therefore, as will be appreciated bythose of ordinary skill in the art in view of the present disclosure,high efficiency combined with high signal accuracy are among theimportant advantages of load isolation.

In particular, the power amplifier device disclosed herein addressessome of the above-noted design limitations of the prior art byeliminating undesirable effects of finite transition time, and byproviding higher efficiencies and other possible performance advantagesrelative to conventional switching mode power amplifiers. The poweramplifier disclosed herein also provides load isolation in two manners:by isolating the load via switching of solid-state semiconductorswitches only at detected zero-crossings of an input signal, and byisolating the same load from a power supply via the use of transformers,e.g., coupled inductors or forward converters, having a high degree ofcoupling resulting in low leakage inductance and thus higher efficiency.

As part of a disclosed embodiment, positive and negative components of amodulated signal are processed separately through different transformersand solid-state semiconductor switches. Energy transfer to and fromprimary windings of the transformers is controlled via another set ofsolid-state semiconductor switches and diodes as set forth herein. Agoal of the present disclosure is to produce high efficiency signalamplification with low levels of signal distortion. The impedance load,the identity of which may vary with the intended application, may bevariously embodied by way of non-limiting examples as audio speakers,transducers, electric motors, and/or any other suitable load requiringan amplified input signal for operation.

In an example embodiment, a power amplifier device for delivering powerto a load includes first and second pairs of switches, a pair oftransformers, and a zero-crossing circuit. The respective switches ofthe first pair of switches receive a respective positive and a negativecomponent of a modulated input signal formed from an analog input signaland a carrier signal. A switching rate of the switches in the first pairexceeds a frequency of the carrier signal. The transformers, each ofwhich is electrically connected to a respective switch of the first setof switches, are operable for accumulating energy from the positive andnegative components, respectively.

In a possible non-limiting embodiment, each transformer is operable forreleasing its accumulated energy only when the respective switch of thefirst pair of semiconductor switches is turned off.

The switches of the second pair of switches have opposite switchingstates. Each switch is electrically connected between a respective oneof the transformers and the load, and each is operable for receiving thereleased energy from the respective transformers. A switching state ofthe switches in the second pair of switches changes only in response toa detected zero voltage crossing of the input signal. Aclosed/conducting switching state of the second pair of switchestransfers the released energy to the load in this particular embodiment,with the zero-crossing detection circuit operable for detecting the zerovoltage crossing.

The first pair of switches may be optionally embodied as metal-oxidesemiconductor field effect transistors (MOSFETs) and the second pair ofswitches as insulated gate bipolar transistors (IGBTs), without limitingthe switches to such embodiments.

Additionally, a method for delivering power to a load includes receivinga modulated input signal having separate positive and negative voltagecomponents, with the modulated input signal being comprised of a carriersignal having a carrier frequency and an analog input signal having aninput frequency. The method includes directing the positive and negativevoltage components to first and second semiconductor switches,respectively, and then switching the first and second semiconductorswitches at a rate equal to or exceeding the carrier frequency tothereby deliver energy from respective first and second transformers torespective third and fourth semiconductor switches.

The method also includes detecting a zero-crossing of the input signalusing a zero-crossing detector chip or other circuit having a comparatorcircuit, and then selectively opening one of the third and fourthsemiconductor switches and closing the other so as to deliver thereleased energy to the load only when the zero-crossing is detected.

A system is also disclosed herein having a modulation circuit operablefor receiving and modulating an analog input signal, a load, and a poweramplifier device. The modulation circuit is operable for generating amodulated input signal from a carrier signal and an analog input signalusing a ternary modulation technique such as pulse width modulation orpulse density modulation.

The above and other features and advantages of the present disclosurewill be readily apparent from the following detailed description of theembodiment(s) and best mode(s) for carrying out the described inventionwhen taken in connection with the accompanying drawings and appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic circuit diagram of an example embodiment of aswitching mode power amplifier as described herein.

FIG. 1A is a schematic circuit diagram of an alternative embodiment ofthe switching mode power amplifier shown in FIG. 1.

FIG. 2 depicts an example modulation pulse train and resultantintegrated signal having positive and negative components.

FIG. 3 is a time plot of an example input signal having zero voltagecrossing points used to control switch timing of a portion of thecircuit shown in FIG. 1.

FIG. 4 is a schematic flow chart describing an example method for usingthe switching mode power amplifier shown in FIG. 1.

DETAILED DESCRIPTION

Referring to the drawings, wherein like reference numbers refer to likecomponents, a system 50 is depicted that includes a switching mode poweramplifier device 10 and an example load 30 represented schematically asa resistor R_(L). The power amplifier device 10 is operable foramplifying a received modulated signal, and for ultimately powering theload 30 within the system 50 using energy released from the amplifiedmodulated signal. The system 50 also includes a modulation circuit 16A,16B operable for receiving an input signal 40, e.g., information encodedas a periodic or other time-varying signal, modulating the receivedinput signal 40 using a carrier signal (arrow C), and outputtingseparate first and second modulated voltage signals (V_(M) ⁺, V_(M) ⁻)as respective positive and negative voltage components. While shown asseparate elements 16A (MOD⁺) and 16B (MOD−) to indicate separateprocessing of positive and negative cycles of a waveform defining theinput signal 40, the modulation circuit 16A, 16B may be embodied as asingle integrated circuit outputting the separate first and secondmodulated voltage signals (V_(M) ⁺, V_(M) ⁻) via different pins (notshown), as will be appreciated by one of ordinary skill in the art.

Within the system 50 of FIG. 1, the power amplifier device 10 includesfirst and second pairs of solid-state/semiconductor switches, i.e., afirst switching pair formed respectively of a first and secondsemiconductor switch S1 and S2, and a second switching pair formedrespectively of a third and fourth solid-state semiconductor switches S3and S4, as well as a pair of transformers 18 formed from a first andsecond transformer T1 and T2 and a pair of steering diodes 19 formedfrom a respective first and second steering diode D1 and D2, all ofwhich are described in further detail below. FIG. 1A depicts alternativetransformers 180 in an uncoupled inductor alternative embodiment, withthe battery 12 and input signal 40 omitted from FIG. 1A for simplicity.Additionally, all switches S1, S2, S3, and S4 of FIG. 1A are depictedschematically as boxes to indicate that any combination of suitablesemiconductor switches may be used, such as the IGBTs and MOSFETs ofFIG. 1 or other configurations as set forth herein. A zero-crossingdetection (ZCD) circuit 20 in the form of an integrated circuit or othersuitable structure is also used as part of the power amplifier device10, receiving the input signal 40 as an input voltage VIN and providingseparate outputs ZC+ and ZC− to respective solid-state switches S3 andS4 of the type described below. In some designs, the modulation circuit16A, 16B may be positioned separately from the power amplifier device 10without departing from the intended inventive scope.

With respect to the structure and intended function of each component ofthe power amplifier device 10 or the larger system 50 within which thepower amplifier device 10 is used, a direct current (DC) battery (B) 12may be embodied as a multi-cell battery of the type known in the art.The battery 12 has a calibrated DC voltage output level, e.g.,approximately 120-150 VDC in an application-specific example embodimentin which the load 30 is a high-voltage transducer, e.g., for a sonobuoyor other device. As noted above, the modulation circuit 16A, 16B may beembodied as a single integrated chip or other circuit device althoughshown as separate elements for illustrative clarity.

The modulation circuit 16A, 16B is operable for modulating the receivedinput signal 40 using the carrier signal (arrow C), e.g., viaapplication of a periodic triangle wave, a square pulse wave, or othertypical periodic or repeating modulation waveform. The terms“modulation” and “modulating” as used herein refer to any suitableternary modulation technique, such as but not limited to pulse widthmodulation (PWM) or pulse density modulation (PDM). The term “ternary”as used herein means that the modulated signal has only three possiblestates, i.e., positive, zero, and negative voltage states. Themodulation circuit 16A, 16B of FIG. 1 outputs the separate first andsecond modulated voltage signals (V_(M) ⁻, V_(M) ⁺), with a zero voltagecrossing between the first and second modulated voltage signals (V_(M)⁺, V_(M) ⁻) being automatically detected via operation of thezero-crossing detection circuit 20 and used in timing of the powerswitching control of the second pair of semiconductor switches S3 and S4as explained below.

Referring briefly to FIG. 2, ternary modulation used herein can bedescribed via the example use of center-sampled modulation pulses 25shown as progressively wider modulation pulses numbered 1, 2, 3, and 4.A waveform 35 demarcated by the same pulse numbers 1, 2, 3, and 4depicts an integrated voltage (V) delivered over time (t) by thesuccessive modulation pulses 25, and also depicts the ternary aspect ofmodulation noted above, i.e., with separate positive (+), negative (−),and zero (0) voltage levels. The modulation circuit 16A, 16B of FIG. 1separates the positive (+) and negative (−) components, whichrespectively correspond to the first and second modulated voltagesignals (V_(M) ⁺, V_(M) ⁻) described above. The modulated voltage signal(V_(M) ⁺) is then delivered to the primary windings of the firsttransformer T1 of FIG. 1 while the first semiconductor switch S1 ison/conducting. Similarly, the modulated voltage signal (V_(M) ⁻) isdelivered to the primary windings of the second transformer T2 of FIG. 1when the second semiconductor switch S2 is on/conducting.

The term “transformer” as used herein with reference to the examplefirst and second transformers T1 and T2 refers to a set of primary andsecondary inductor windings, whether configured as an optional coupledinductor as shown in FIG. 1 or as an uncoupled inductor as shown in FIG.1A. As is known in the art, coupled inductors such asflyback convertersstore electrical energy from a power source in a primary windingwhenever a semiconductor switch supplying power to the coupled inductoris turned on/conducting, but do not transfer stored energy to asecondary winding until the same switch is turned off. In a forwardconverter as typically used in a distributed power architecture, energyis transferred from the primary to the secondary winding while theswitch supplying the primary winding is on/conducting. Of the varioustypes of transformers in use, coupled inductors are specificallydesigned and intended to store energy, and therefore may provide certainperformance advantages when used within the system 50 depicted inFIG. 1. However, coupled inductors are just one possible way toimplement the disclosure.

In one of the possible embodiments, energy can be stored via thetransformers T1 and T2 only when electrical current flows in the primarywindings of the transformers T1 and T2. With power defined as energydelivered per unit time, power delivered to the load 30 is thusdependent on the rapid transfer of energy through the power amplifierdevice 10. In other words, if electrical current can be pushed morequickly through the primary windings of the transformers T1 and T2 ofFIG. 1, more energy is ultimately produced as a function of energy (E),inductance (L), and electrical current (i), with this functionrepresented mathematically as E=½Li².

As will be appreciated by those of ordinary skill in the art, theequation noted above is a solution to an integral, and thus establishesthat an inductor acts an integrator of current. The integration time ismuch longer than the transition time of a semiconductor switch. That is,voltage across an inductor is expressed as

$V = {L{\frac{d\; i}{d\; t}.}}$The increase of energy over time is expressed as. Thus, the energystored in an inductor is expressed as

${E = {{\int{d\; E}} = {{L{\overset{i}{\int\limits_{0}}{i\; d\; i}}} = {\frac{1}{2}{Li}^{2}}}}},$with the time associated with overall switching function being afunction of inductor switching. This in turn illustrates the linearitybenefit of the present approach, as the ratio of the semiconductortransition time to the inductor current integration. time is a verysmall number, and therefore contributes very little to distortion. Inaddition, at low power, in a PWM system small pulse widths are needed,with wider pulse widths needed at high power. This means that staticlosses accumulate only as a function of power storage. The fact the loadis isolated during this interval means that the only load during thecharge interval is the inductor current.

The power amplifier device 10 of FIG. 1 is thus scalable to larger orsmaller loads 30, with isolation of the load 30 provided in twodifferent manners: via switching isolation due to the control of thesecond set of switches S3 and S4 only at the zero-crossing of the inputsignal 40, and via low levels of leakage inductance provided by thetransformers T1 and T2.

Referring again to FIG. 1, the switches S1 and S2 respectively receivethe corresponding voltage signals V_(M) ⁺ and V_(M) ⁻, and then transferthe voltage signals V_(M) ⁺ and V_(M) ⁻ to the respective first andsecond transformers T1 and T2. The semiconductor switches S3 and S4 areelectrically connected to an output side of the first and secondtransformers T1 and T2, respectively, and thus are powered by thetransformers T1 and T2 according to a particular switching controlsequence as set forth below with reference to FIG. 4. The diode D1 inthis embodiment is serially connected, i.e., connected in electricalseries, between the first transformer T1 and the third semiconductorswitch S3, while the diode D2 is serially connected between transformerT2 and the fourth semiconductor switch S4.

In the example embodiment as shown in FIG. 1, the first pair ofsemiconductor switches S1, S2 may be optionally embodied in anon-limiting application as metal-oxide semiconductor field effecttransistors (MOSFETs) of the type known in the art, with the second pairof semiconductor switches S3, S4 may be optionally embodied as insulatedgate bipolar transistors (IGBTs). As is known in the art, IGBTs havedesignated gate (G), emitter (E), and collector (C), each of which islabeled as such in FIG. 1. However, other solid-state switchconfigurations may be used depending on the intended application,including using only MOSFETs, only IGBTs, or using other gate-controlledsolid-state switches. For instance, high electron mobility transistors(HEMTs) using gallium nitride (GaN) or other suitable materials may beused in lieu of MOSFETS and IGBTs. In general, the switching rate andthus the design of the first pair of semiconductor switches S1, S2depends on the modulation encoding scheme used by the modulation circuit16A, 16B.

As a design consideration, within the scope of the present disclosurethe first pair of semiconductor switches S1, S2 has a high switchingspeed requirement in that the first pair of semiconductor switches S1,S2 must always switch at the or above the frequency of the carrier (C),i.e., the carrier frequency. By contrast, the switches of the secondpair of semiconductor switches S3, S4 change their respective switchingstates at a substantially slower rate than that of the first pair ofsemiconductor switches S1, S2, i.e., switching only at the zero-crossingrate of the input signal 40, with the carrier frequency expected hereinto be substantially higher than the zero-crossing rate.

To illustrate the latter point, FIG. 3 depicts the input signal 40 as atime-varying signal, i.e., having a voltage magnitude V, that changesover time (t). The input signal 40 has positive (+) and negative (−)components as shown. The input signal 40 crosses through zero voltswhenever the input signal 40 changes its sign, with each zero-crossingdemarcated in FIG. 3 by a corresponding zero-crossing point 42. Thezero-crossing detection circuit 20 of FIG. 1, for instance anoperational amplifier or other suitable comparator circuit or otherintegrated circuit, detects each zero-crossing point 42 and, in responseto such detection, activates a designated one of the third or fourthsemiconductor switches S3 or S4 and simultaneously deactivates the othersemiconductor switch S3 or S4 in the same pair, with the identity of theactivated switch changing with the detection of each successivezero-crossing point 42.

That is, when the third semiconductor switch S3 of FIG. 1 is activelyconducting, the fourth semiconductor switch S4 is not conducting, andvice versa. In this manner, the switching function of the first pair ofsemiconductor switches S1, S2 is temporally separated from anydownstream switching used to deliver stored energy from the transformersT1 or T2 to the load 30, i.e., the switching function in the switchingmode power amplifier device 10 is isolated from its load function viatargeted low-frequency/zero-crossing control of the second pair ofsemiconductor switches S3, S4 and diodes D1, D2.

As part of the power amplifier device 10, the steering diodes D1 and D2direct the energy released by the transformers T1, T2 to the second pairof switches S3, S4. The diodes D1, D2 are thus an important part of theload isolation functionality enabled by the present disclosure, and forthat reason should be constructed from materials of sufficientlyhigh-speed and high-energy density, such as silicon carbide, galliumnitride, or other high-mobility semiconductor materials. With respect tothe transformers T1 and T2 of FIG. 1, these devices may be embodied assuitable energy storage devices, such as those having a linear output asa function of pulse density or pulse width. In the embodiment of FIG. 1,energy is stored in the primary winding of each of the transformers T1and T2 when the corresponding semiconductor switch S1 or S2 ison/conducting, and is transferred to the secondary winding only when thecorresponding semiconductor switch S1 or S2 is turned off, i.e., is notconducting.

The transformers T1, T2 should be constructed in such a manner as toprovide high-quality inductive coupling between the primary andsecondary windings and thereby provide low levels of leakage inductance.As used herein, “low leakage inductance” refers to levels of less thanabout 1/80^(th) to 1/100^(th) of a primary inductance of the primarywindings. In another optional embodiment, the primary and secondarywindings of each of the transformers T1 and T2 may be concentricallywound, i.e., the primary winding of the first transformer T1 is woundconcentrically with the secondary winding of the second transformer T1,with the same arrangement in the second transformer T2. Additionally,the transformers T1 and T2 are electrically connected in reversepolarity with respect to each other.

The identity of the load 30 shown schematically in FIG. 1 may vary withthe particular application. In general, in order to benefit fully fromthe present disclosure, the load 30 may be a high impedance reactiveload having a power factor exceeding 0.65 for high-power applications.Example embodiments of the load 30 may include a transducer, an antenna,an audio speaker, or an electric motor. In a particular embodiment, theload 30 may be a sonobuoy transducer used as part of a sonobuoyassembly, for instance to deploy or actuate directional hydrophones orother aquatic acoustic sensors and/or signal transmitters, without inany way limiting applications to such a field of art.

Referring to FIG. 4, an example embodiment of the method 100 begins withstep S102, which may be executed offboard with respect to the remainingsteps of the method 100. At step S102, the input signal 40 of FIG. 1 ismodulated. The input signal 40 may be transmitted from a remote locationsuch as a transmission tower or antenna and communicated to themodulation circuit 16A, 16B as is well known in the art. The modulationcircuit 16A, 16B itself may be separate from the switching mode poweramplifier device 10. Step S102 may entail modulating the input signal 40via a carrier signal (arrow C) using ternary PWM, ternary PDM, or otherconventional ternary modulation techniques, doing so via operation ofthe modulation circuit 16A, 16B of FIG. 1. The method 100 proceeds tostep S104 once the input signal 40 has been modulated.

At step S104, the modulated signal that is output by the modulationcircuit 16A, 16B is separated into its positive and negative voltagecomponents (V_(M) ⁺, V_(M) ⁻) as shown in FIG. 2. In practice, themodulation circuit 16A, 16B may be designed to output separate positiveand negative voltage components (V_(M) ⁺, V_(M) ⁻) as opposed toseparating a modulated signal into the different components. Thepositive voltage component (V_(M) ⁺) is then electrically conducted ortransmitted to the gate (G) of the first semiconductor switch S1 shownin FIG. 1. Likewise, the negative voltage component (V_(M) ⁻) isdelivered to the gate (G) of the second semiconductor switch S2. Themethod 100 then proceeds to step S106.

Step S106 entails switching the first set of semiconductor switches S1,S2 at a rate that is equal to or greater than the carrier frequency(f_(C)). For example, if the carrier frequency is 30-40 kHz, theswitching frequency of the first pair of semiconductor switches S1, S2may be at least 30-40 kHz, or approximately 350-400 kHz or about 10times the carrier frequency in other embodiments. Only when the first orsecond semiconductor switch S1 or S2 is commanded to an on/conductingstate will energy be stored in the primary windings of the transformersT1 or T2, respectively. The method 100 then proceeds to step S108.

Step S108 entails detecting a zero voltage crossing of the input signal40 via the zero-crossing detection circuit 20 of FIG. 1, with examplezero-crossing points 42 depicted in FIG. 3. Step S108 may include usingan operational amplifier or other suitable comparator circuit to detectthe zero voltage crossing. The method 100 proceeds to step S110 when azero voltage crossing is not detected, and to step S112 when a zerovoltage crossing is detected.

At step S110, the present switching state of the second pair ofsemiconductor switches S3, S4 is maintained, i.e., not changed. Forinstance, if the third semiconductor switch S3 is on/conducting and thefourth semiconductor switch S4 is off/not conducting, then switch S3remains on and switch S4 remains off. The method 100 proceeds to stepS114.

Step S112 includes changing the switching state of the semiconductorswitches S3, S4 from a state that existed just prior to the detection ofa zero voltage crossing at step S110. For example, if the thirdsemiconductor switch S3 was on/conducting and the fourth semiconductorswitch S4 was off/not conducting, the detection of a zero voltagecrossing at step S108 results in the third semiconductor switch S3turning off and the fourth semiconductor switch S4 turning on, forinstance via a change of voltage delivered to the gates (G) shown inFIG. 1. The method 100 then proceeds to step S114.

At step S114, electrical power or energy is delivered to the load 30through the on/conducting semiconductor switch S3 or S4, whichever ofthe two is in a conducting state.

As set forth above, the power amplifier device 10 provides for signalconversion and amplification via the use of the transformers T1, T2 andtargeted low-frequency switching control of the second pair ofsemiconductor switches S3, S4 downstream of the transformers T1, T2.This occurs only at detected zero voltage crossing points of an inputsignal, such as the zero voltage crossing points 42 and input signal 40of FIG. 3, to allow the load 30 and its associated impedance to beisolated from the high-speed switching function of the first pair ofsemiconductor switches S1, S2. As a result, the first pair ofsemiconductor switches S1, S2 is able to work more efficiently thanwould ordinarily occur in existing designs, with gains of 30% inefficiency or more being possible relative to the typical efficiencylevels available via the conventional art. Also, the power amplifierdevice 10 allows for a fully scalable power output, with thehigh-efficiency control technique of the method 100 minimizing internaldissipation of heat within the power amplifier device 10. Such benefitsmay be desirable in many applications, including but not limited totransducer systems of the type used for coupling acoustic pulse energyinto water, e.g., sonobuoy applications, audio applications in whichmodulated waves are transmitted to an antenna for playback via a set ofspeakers, and the like.

The detailed description and drawings are supportive and descriptive ofthe disclosure, but the scope of the invention is defined solely by theclaims. While some of the best modes and other embodiments for carryingout the disclosure have been described in detail, various alternativedesigns and embodiments exist for practicing the disclosure as definedin the appended claims.

What is claimed is:
 1. A switching mode power amplifier device fordelivering power to a load, the switching mode power amplifier devicecomprising: a first pair of semiconductor switches comprising first andsecond semiconductor switches each configured to receive a modulatedinput signal from a modulating circuit, the modulated input signal beingcomprised of a time-varying analog input signal and a carrier signalhaving a carrier frequency, the modulated input signal having modulatedpositive and negative voltage signals respectively corresponding topositive and negative components of the time-varying analog inputsignal, the time-varying analog input signal changing between thepositive and negative components at a zero-voltage crossing of thetime-varying analog input signal, wherein a switching rate of the firstand second semiconductor switches equals or exceeds the carrierfrequency of the modulated input signal; first and second transformersrespectively connected to the first and second semiconductor switches,and respectively receiving the modulated positive and negative voltagesignals from the first and second semiconductor switches, the first andsecond transformers being configured to store energy from the modulatedinput signal; a second pair of semiconductor switches comprising thirdand fourth semiconductor switches that are electrically connectedbetween a respective one of the first and second transformers and theload, wherein a switching state of the third and fourth semiconductorswitches changes in response to an output signal indicative of thezero-voltage crossing of the time-varying analog input signal, andwherein a closed/conducting switching state of the third and fourthsemiconductor switches transfers the energy stored in the first andsecond transformers to the load, such that the load is isolated from aswitching function of the first pair of semiconductor switches; a firstdiode positioned in series between the first transformer and the thirdsemiconductor switch; a second diode positioned in series between thesecond transformer and the fourth semiconductor switch; and azero-crossing detection (ZCD) circuit in communication with the secondpair of semiconductor switches, the ZCD circuit being operable fordetecting the zero-voltage crossing of the time-varying analog inputsignal and for transmitting the output signal indicative of the detectedzero-voltage crossing to the third or fourth semiconductor switches. 2.The switching mode power amplifier device of claim 1, wherein the loadhas a power factor not less than 0.65.
 3. The switching mode poweramplifier device of claim 1, wherein the first and second semiconductorswitches are metal-oxide semiconductor field effect transistors and thethird and fourth semiconductor switches are insulated gate bipolartransistors.
 4. The switching mode power amplifier device of claim 1,further comprising the modulation circuit.
 5. The switching mode poweramplifier device of claim 4, wherein the modulation circuit isconfigured to generate the modulated input signal via ternary modulationof the carrier signal.
 6. The switching mode power amplifier device ofclaim 5, wherein the ternary modulation includes pulse densitymodulation or pulse density modulation.
 7. The switching mode powersupply of claim 1, wherein each of the first and second transformers hasconcentrically-wound primary and secondary windings.
 8. A method fordelivering power to a load using a switching mode power amplifier devicehaving first, second, third, and fourth semiconductor switches and firstand second transformers, the method comprising: receiving a modulatedinput signal from a modulating circuit, the modulated input signalhaving separate modulated positive and negative voltage signalscorresponding to positive and negative components of a time-varyinganalog input signal, wherein the modulated input signal is comprised ofthe time-varying analog input signal and a carrier signal having acarrier frequency; directing the modulated positive and negative voltagesignals to the first and second semiconductor switches, respectively;switching the first and second semiconductor switches at a switchingrate that equals or exceeds the carrier frequency to thereby storeenergy from the respective modulated positive and negative voltagesignals in the first and second transformers; detecting a zero-voltagecrossing of the time-varying analog input signal using a zero-crossingdetector (ZCD) circuit, wherein the time-varying analog input signalchanges between the positive and negative components at the zero-voltagecrossing; generating an output signal of the ZCD circuit indicative ofthe zero-voltage crossing; and selectively opening one of the third andfourth semiconductor switches and closing the other of the third andfourth semiconductor switches responsive to the output signal from theZCD circuit to thereby deliver the energy stored in the first or secondtransformer, through a respective first or second diode, through therespective third or fourth semiconductor switch, and to the load inresponse to the zero voltage crossing, such that the load is isolatedfrom a switching function of the first and second semiconductorswitches.
 9. The method of claim 8, wherein the load is a transducer, anaudio speaker, or a motor.
 10. The method of claim 9, wherein the loadis the transducer, and the transducer is a sonobuoy transducer.
 11. Themethod of claim 8, further comprising generating the modulated inputsignal via ternary modulation of the carrier signal using the modulationcircuit, wherein the ternary modulation of the carrier signal includespulse width modulation or pulse density modulation of the carriersignal.
 12. The system of claim 8, wherein the modulation circuit isconfigured to generate the modulated input signal via ternary modulationof the carrier signal, including pulse width modulation or pulse densitymodulation of the carrier signal.
 13. A switching mode power amplifierdevice for delivering power to a load, the switching mode poweramplifier device comprising: first and second semiconductor switchesconfigured to respectively receive a modulated positive voltage signaland a modulated negative voltage signal from a modulating circuit, themodulated positive and negative voltage signals being comprised of acarrier signal and respective positive and negative voltage portions ofa time-varying analog input signal, the carrier signal having a carrierfrequency that is less than a switching rate of the first and secondsemiconductor switches; a zero-crossing detection (ZCD) circuit incommunication with the third and fourth semiconductor switches, the ZCDcircuit being configured to detect a zero-voltage crossing of thetime-varying analog input signal, and to transmit an output signalindicative of detection of the zero-voltage crossing to one of the thirdor fourth semiconductor switches; first and second inductorsrespectively connected to the first and second semiconductor switches,and respectively receiving the modulated positive and negative voltagesignals from the first pair of semiconductor switches, wherein the firstand second inductors store energy from the modulated positive andnegative voltage signals, respectively, when the first or secondsemiconductor switches have an on/conducting state; third and fourthsemiconductor switches electrically connected between a respective oneof the first and second inductors and the load, wherein a switchingstate of the third and fourth semiconductor switches is configured tochange to a closed/conducting state responsive to the output signal fromthe ZCD circuit, and wherein the closed/conducting switching statetransfers the energy stored in the first and second inductors to theload, such that the load is isolated from a switching function of thefirst pair of semiconductor switches; a first steering diode positionedin series between the first inductor and the third semiconductor switch;and a second steering diode positioned in series between the secondinductor and the fourth semiconductor switch.
 14. The system of claim13, wherein the first and second inductors are coupled inductors. 15.The system of claim 13, wherein the first and second inductors areuncoupled inductors.
 16. The system of claim 13, wherein the first andsecond semiconductor switches are metal-oxide semiconductor field effecttransistors and the third and fourth semiconductor switches areinsulated gate bipolar transistors.